1. Field of the Invention
The present invention relates to an image forming apparatus comprising a structure to prevent a sheet fixed by a fixing device of the image forming apparatus for forming an image by an electrophotographic system from curling, and a sheet conveying method of the image forming apparatus.
2. Description of the Related Art
Conventionally, in the electrophotographic system, a heating and fixing device of a contact heating type in which a toner image on a paper is heated and pressurized to be fixed has been broadly used. Further, a heating and fixing device in which a toner image is heated and fixed while sandwiching and conveying a paper by a fixing roller and a pressure roller has been most widely used.
In the case of heating a paper to fix an image, the paper becomes soft, so that paper deformation, especially curl of the paper easily occurs. Thus, there is a problem that the curl of the paper occurs after passing the heating and fixing device, and in a subsequent conveying path. When the curled paper is conveyed on the conveying path provided on the downstream of the fixing device, problems occur such as that jamming occurs or that papers are not aligned on an eject tray.
As a method to solve the occurrence of the curl of the papers due to such heating and fixing device, there is disclosed a technique in which the curl of the paper which was fixed is corrected by using a conveying belt and a pressure roller (JP-Tokukaihei-5-341600 and JP-Tokukaihei-6-144677, hereinafter refer to “Patent Document 1” and “Patent Document 2”, respectively). Moreover, there is disclosed a technique in which a curl removing member to curve the paper by contacting the surface of the paper which was fixed is provided, and thereby removing the curl which was generated due to the fixing (JP-Tokukaihei-6-115792, hereinafter refer to “Patent Document 3”). Further, there is disclosed a technique in which an upper side sheet path and a lower side paper path are provided on the downstream side of the fixing device, thereby removing the curl in any direction which was generated due to the fixing by switching the paths with the use of a guide member (JP-Tokukaihei-7-61677, hereinafter refer to “Patent Document 4”). Further, there is disclosed a technique in which the curl of the paper is reduced by the combination of controlled surface temperatures of the fixing roller and the pressure roller (JP-Tokukai-2003-302861, hereinafter refer to “Patent Document 5”). The above described techniques of the Patent Documents 1-5 have a technical idea of removing the curl of the paper by correcting the curl generated due to the heating and fixing device.
However, according to the studies by the inventors, the phenomenon has been found out that the final curl of the paper is determined due to the shape of the conveying path provided on the downstream side of the fixing device as well as the curl generated by the fixing device. That is, the paper becomes soft due to the heat by the fixing device to be easily deformed, and the final curl of the paper is determined after it was cooled to some extent. Accordingly, in the case where the conveying path on the downstream side of the fixing device is linearly formed, the curl generated due to the fixing device may be removed. However, in the case where the conveying path on the downstream side of the fixing device is curved, the curl corresponding to the curved shape is further generated. That is, the paper is cooled while being conveyed on the curved shaped conveying path, so that another curl which is different from the curl generated due to the fixing device is generated due to the curved shaped conveying path. The final curl of the paper is considered as the curl due mainly to the curved shaped conveying path.